Exemplary embodiments of the present disclosure encompass, for example, the activation of electropneumatic valves which can be used as actuating valves for the control or position regulation of actuating or regulating drives. Valves of this type can be designed as 3/3-way valves in order to make it possible to, in addition to providing an aerating and venting function, have a closed-off middle position for deviation control according to stipulated desired values, so that, in emergency situations, for example, the current actuating pressure can be kept constant, and the connected actuating drive can thereby remain in its current position. For acting with a pressure medium upon the actuating drive, a booster stage can be provided downstream of the 3/3-way valve and be acted upon by the pilot control pressure. Such a booster stage can be designed, with the effect of an intensifying function, to generate a higher actuating force for a correspondingly higher actuating pressure. Within this scope of such an exemplary field of use, exemplary embodiments of the present disclosure are directed to the behavior of the position controller in the event of a failure of the feed pressure supply, which can provide compressed air, for example, as the pressure medium.
It is known that, in the event of the failure of the feed pressure supply, the valve mechanism comes into an initial position which ensures a venting of the connected pneumatic actuating drive. Venting has the effect that the connected actuating drive is moved into a defined end position via an integrated spring, thus, in turn, completely opening or closing the fitting connected to it. Such a fitting may in this case be, for example, a flat slide valve inserted into a pipeline of a chemical engineering plant.
However, special applications require that the fitting connected to the actuating drive maintain its current position when the feed pressure supply to the position controller or the electrical actuating signal fails. This requirement has hitherto been a substantial reason for the use of electrical position controllers instead of the pressure medium-operated position controllers relevant to the present disclosure.
A generic pressure medium-operated position controller is disclosed in US 2007/0045579 A1. This position controller has an actuating device, by means of which a feed pressure connection, a venting connection and a working connection for generating an actuating pressure of an actuating drive can be switched variably. To stipulate the desired switching position, the actuating device has two fluid action surfaces which are oriented opposite to one another and which each delimit a control chamber. Both control chambers are connected to a common control pressure connection, with a throttle device being interposed. Downstream of the two throttle devices, each control chamber is connected to a venting port. A control valve device can control the two venting ports and also close them simultaneously.
This symmetrical set-up with respect to the two fluid action surfaces, along with activation via one commonly assigned control pressure connection, ensures that the fluidic actuating forces acting on the control device when the two venting ports are closed simultaneously compensatory to one another, and a clearly defined position of the actuating device is obtained. There is the possibility, with the venting ports closed, of stipulating a basic position of the actuating device in which the working connection is separated both from the feed connection and from the venting connection. That is, the middle switching position of a 3/3-way valve can be achieved, so that a constant pilot control pressure is maintained, to thereby give the position controller a blocking fail-safe behavior.
The electrical activation of the position controller may take place in a flexible way so that it is possible to control the downstream booster stage such that (1) the pressure medium can be conducted in a directed manner from the feed pressure connection via the working connection into the pneumatic actuating drive, or (2) the pressure medium can be conducted in a directed manner out of the pneumatic actuating drive via the venting connection into the atmosphere, or (3) the pressure medium can be enclosed in the booster stage to maintain the current position of the actuating drive.
In this known technique, however, the fail-safe behavior as a result of the failure of pressure medium (e.g., due to the breakaway of the pneumatic feed pressure line from the feed pressure connection) is a disadvantage. This is because, depending on the electrical activation prevailing at this timepoint, the electropneumatic valve functioning as a pilot control valve will fail either when venting or when blocking. The choice in this case is, as far as possible, left to chance. Venting failure means that the pressure medium is discharged out of the actuating drive into the surroundings. Blocking failure means that the pressure medium contained in the booster stage, i.e., in the actuating drive, is enclosed.
The reason for this weakness in the system is that the feed pressure medium supplied to the control valve device is extracted from the feed pressure duct of the position controller. When the position controller assumes the aerating position for the connected actuating drive, the assigned valve chamber opens and thus connects the feed pressure connection to the feed pressure chamber for acting upon the actuating drive.
Then, if the feed pressure line is separated, with electrical activation unchanged, the actuating drive is capable, via the spring return position integrated in it, of venting and/or ventilating the compressed air contained therein via the open feed pressure connection. However, since the control valve device continues to remain regulatable with the aid of the outflowing compressed air, the control pressure is still maintained, and therefore the open position of the position controller can continue to be maintained. The actuating drive is therefore vented until the control pressure controller no longer delivers sufficient control pressure, at which time the position controller is finally closed. The position drive is then vented completely and is in the pressureless initial position.
If, by contrast, the control valve device is activated such that a venting or blocking of the actuating drive is brought about, the pressure medium is enclosed in the actuating drive, even with the feed pressure connection separated, and therefore the actuating drive is blocked.